Common causes of rough idle on 1.8T's are. Failing coolant temp sensor, Gunked up throttle body, split vacuum/boost pipes. Scan with VCDS is useful, but may not show any fault codes relating to the aforementioned. Sounding like a deisel on cold atart up is usually caused by the Secondary air intake housing rivets failing. Noise should stop within 90 secs if it doesn't then you may have oil starvation going on, which is often caused by the Sump oil pick up being blocked with sludge.

Just so you know. The CTS has 2 independent outputs 1 to the Dash Temp gauge & the other to the ECU. If the ECU side is sending out incorrect temp to the ECU it will not fuel correctly hence poor idle. It is quite normal for one CTS output to work normally while the other is failing.
A lot of people decide to buy aftermarket CTS's as they are much cheaper than the genuine VAG part, problem is that rarely do the cheap CTS's work correctly for long or indeed at all. Buy cheap buy twice.

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